Evolutionary Arms Races Flashcards
Types of prey defenses
Predator adaptations and prey counter-adaptations: searching for prey
Predator adaptations:
- develop a ‘search image’
- Improved sensory capacity
- Focus search on small area
Prey counter-adaptations:
- space out behaviorally
- crypsis
- polymorphic appearance
Predator adaptations and prey counter-adaptations: recognizing prey
Predator adapations:
- learning
Prey coubnter-adaptaions:
- warning signs of toxicity
- masquerade
- deception by mimicry
Predator adaptations and prey counter-adaptations: catching prey
Predator adaptations:
- secretive approach (motor skills, speed, agility)
- offensive weapons
Prey counter-adaptations:
- startle response (e.g. eyespots)
- signal to predator that it’s been detected
- escape flight
Predator adaptations and prey counter-adaptations: handling/eating prey
Predator adaptations:
- ‘subduing ability’
Prey counter-adaptations:
- toxicity
- e.g. spikes
Red queen evolution
If predators adapt and prey counter-adapt, does anyone ever really have the advantage?
Crypsis in moths
- Cryptic-looking forewings (for camouflage?) but brightly-colored hindwings
- Gave a slideshow to bluejays in an aviary
- Backgrounds were cryptic or conspicuous
- In some cases, a moth was present in the picture, in others not
- If a moth was present, birds could get a mealworm as a reward for pecking at the slide
- If no moth was present, the jay would peck a key to advance to another slide
Search images
- Luc Tinbergen: birds would not eat certain insects each year when they appeared in the spring, but started to include them in their diet with tyme
- Hypothesis: change due to an improvement over time in the birds’ ability to see the cryptic insects, aka ‘adopting a specific search image’
-Dawkins’ experiments gave support for this, using chicks and colored rice grains
- Orange grains on green or orange background
- Chicks found prey quicker on a conspicuous background (orange on green)
- On cryptic backgrounds (orange on orange), birds pecked at stones but after 3-4 minutes, located the cryptic grains and started to eat them
- By the end of the trial, ate both grain colors at equal rates on each background
- Supports the hypothesis that birds developed a ‘search image’ over time for cryptic prey
Search images and polymorphic prey
- Some prey are what we call ‘polymorphic’ – different color morphs coexist in the same population
- One hypothesis for polymorphism is that it disrupts search images – if a predator develops a search image for one morph, it may miss other morphs
- Pietrewicz and Kamil tested this with their bluejay setup: bluejays got better and better at locating Morphs A and B when presented alone, but did not improve performance when morphs were mixed
Examples of camouflage beyond background-matching
- Disruptive coloration
- Countershading
- Masquerade
Disruptive coloration
- Bold, contrasting patterns on the periphery of the body, which serve to break up the body outline
- Experiments with fake moths show that cryptic prey survive better than non-cryptic prey
- But disruptive markings enhance survival further
Countershading
- Darkening the dorsal (top) surface
- Lightening the ventral (bottom) surface
- Especially effective in marine or arboreal environments
Masquerade
- Resemblance of inedible objects
- Twigs, leaves, bird droppings, flowers, stones
- Camouflage without crypsis: organism may be detected but is not detected as prey!
- This is difficult to test experimentally – need to demonstrate that the animal was detected but not recognized
Testing masquerade
- Skelhorn et al. used domestic chicks (predator) with twig-resembling caterpillars (prey) placed in clear view)
- Birds in two treatment groups:
- 1) Encountered natural twigs
- 2) No experience with twigs
- Birds with prior experience of natural twigs took longer to attack the caterpillars
- Shows that caterpillars were detected, but misidentified = masquerade
Startle: dazzle and eyespots
- Coloration that creates a startling effect in the predator
- Startles or confuses the predator long enough that the prey can make a getaway
Warning coloration: aposematism
- Some prey are really brightly colored, instead of cryptic
- Why? Doesn’t this make them really easy for predators to spot?
- Certain bright colors (red, yellow, or orange, often combined with black) are associated with defenses such as toxins, spines, or stings
Do prey with defenses really benefit from being conspicuous?
-The desert locus exists in two morphs:
- When populations are low –> cryptic coloration
- Behaviorally, avoid one another, avoid feeding on plants with defensive chemicals
-When population densities are high –> crypsis no longer possible due to not enough habitat for background-matching –> adopt yellow and black coloration
- Behaviorally, group together (gregarious) and start to eat toxic plants to sequester toxic compounds
Sword et al:
-Fed lizards a distasteful, gregarious phase locust
-The next day, fed them another locust from a given treatment group
-Looked at how many lizards rejected the prey given prior experience with aposematic, toxic grasshoppers
- After one trial, predators learned to avoid unpalatable locusts when they are in their black and yellow color morph, rather than cryptic morph
- The change to aposematic is adaptive and reduces predation
The evolution of warning coloration
-Which came first: distastefulness or bright colors?
-Either way poses a paradox:
- Say a mutation arises that makes a prey animal brightly colored
- Suddenly the prey is more obvious to predators –> more likely to perish
- If it was distasteful first: the predator may decide not to attack that kind of prey again, but if the mutation was rare, the mutation goes extinct and the predator never encounters it again
- If it was colorful first: the predator doesn’t learn anything to associate the colors with distastefulness, and may attack others with that mutation
- Thus, even though there is an advantage with predators, the mutation never has a chance to spread because the bright colors make the prey obvious
- Ronald Fisher proposes a solution
Fisher’s solution to questions about the evolution of warning coloration
-Fisher noticed that many aposematic species lived in groups, whereas cryptic species do not
-Proposed that warning coloration can evolve if it arises in family group:
- Siblings are warning colored
- Even if the sampled individual perishes, the siblings are likely to benefit from reduced predation and the gene can spread
- Group selection
-Mathematical models support this, but reality challenges its underlying assumptions
Challenges to Fisher’s assumptions
1) The ‘sampled’ (or attacked) individual always perishes
- False!
- Many bright-colored animals have additional defenses (e.g. hairs) that mean they don’t necessarily die from a predator attack
- Thus, there may be an advantage to the individual of warning coloration if it prevents subsequent attacks by a predator
2) Family grouping sets the stage for the evolution of warning colors
- In some groups (e.g. butterflies), warning coloration evolved before group living
- Thus, there may be an advantage to the individual of grouping because ti means the likelihood of an attack on any given individual is less likely
Explain why mimicry of something distasteful is beneficial
- If one species evolves warning coloration, it benefits from reduced predation
- Selection could then favor the other species to mimic the first species, to also benefit from reduced predation
- The association between bright colors and repellent defenses has led to the evolution of two forms of mimicry
Müllerian mimicry: repellent species look alike
- Fritz Müller: noticed
similarity in color patterns between different repellent species - Hypothesized that if repellent species look alike, it will be easier for a predator to learn to avoid them all
- The predator has to learn just one color pattern for all species to benefit
Müllerian Mimicry: Heliconius butterflies
- Heliconius butterflies:
- species living in the same geographic area share
the same color pattern - different geographic areas have different color
patterns - Benson:
- H. erato and H. melpomene
- Painted some H. erato individuals to be nonmimetic
- Survived less well than controls which were also
painted but remained mimetic - Mallet and Barton:
- Moved butterflies between geographic areas so
that in a given area they moved in some mimetic
and some non-mimetic individuals - Non-mimetic individuals survived less well
Batesian mimicry: cheating by palatable species
- Bates noticed that sometimes, palatable species (mimics) closely resembled distasteful species (models)
- Predators learned to avoid noxious models and subsequently avoid palatable mimics
- Mimics survive better the more closely they resemble the model
Costs of aposematism
- Bright colors can be costly to ‘wear’ - for example, some coloration can interfere with thermoregulation
- Bright colors can be costly to make: toxicity often comes from consuming toxic plants, so the toxic animals themselves must maintain defenses against those toxins, which can take energetic resources
Costs of crypsis
- Being cryptic might interfere with other behaviors (you want to be conspicuous to mates, for example)
Brood parasites
- Exploit the parental care of other species
- Lay their eggs in a host next
- Trick other birds into incubating their eggs and raising their young
- When successful, brood parasites get to have their young raised for free
- Selection for successful parasitism is strong
- Successful parasitism results in zero reproductive success for the host –> selection against successful parasitism is strong
Hosts fight back against brood parasitism: co-evolutionary arms race
- Cuckoos have evolved in response to hosts
- Physical appearance mimics that of a bird of prey
- Egg laying tactics
Cuckoo egg laying tactics
- Fast-laying
- Usually remove one egg when they parasitize a nest
- They time laying to when hosts would usually lay eggs
How have hosts evolved in response to cuckoos?
Egg rejection ability:
- Species unsuitable as hosts (wrong diet, nest type, etc.) do not reject cuckoo eggs
- Some hosts show extremely refined ability to discrimintate ‘self’ from ‘parasite’ egg and reject the parasite
Egg ‘signatures’
- Hosts also evolve more complex egg ‘signatures’ that say ‘this is my egg’
- Signatures make mimicry more difficult for the cuckoo, since it is harder to produce a good match to any given host
Prediction slide forward
Moving on from eggs… why don’t hosts reject chicks?
- Hosts impring upon their own egg type the first time they lay an egg and subsequently reject odd-looking eggs
- This is not foolproof – sometimes, they reject their own egg if it looks a little odd
- Impringing on a chick could be too mistake-prone and mistakes too costly:
- Chicks change appearance each day
- If you accidentally throw your own chick out, that’s bad!
- If your brood is parasitized, you would imprint on a parasite chick
- At the chick stage, better to accept anything than imprint and reject
